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Figure 6.7 The change in the genotype and allele frequency when there is underdominance for fitness and natural selection acts against individuals with Aa genotypes.

The equilibrium allele frequency depends on the initial allele frequency. Starting below 0.5 populations head toward loss while starting above 0.5 populations go to fixation. There is an unstable equilibrium at an initial allele frequency of exactly 0.5. From any initial allele frequency the population converges on a minimum frequency of heterozygotes. The colored, dashed line in the bottom panel corresponds to the allele frequencies in the top panel. In this illustration waa = waa = 1.0 and wAa = 0.9. Genotype frequencies assume random mating.

referred to as heterozygote advantage, overdominance for fitness, or balancing selection (see Table 6.4), are shown in Fig. 6.8. The top panel shows the frequencies of the three genotypes over time starting from an initial allele frequency of p = 0.05. The heterozygous genotype increases in frequency due to its higher relative fitness. At the same time, the aa homozygote (initially 90% of the population) declines due to its lower viability. Although the relative fitness of the AA homozygote is lower than that of the heterozygote, its frequency increases toward 25% as allele frequencies approach p = q = 0.5 due to the increasing frequency of heterozygotes. The bottom panel shows that for all initial allele frequencies, natural selection causes the population to approach p = q = 0.5.

Overdominance for fitness represents a unique exception for the outcome of natural selection on a diallelic locus. Selection against a dominant pheno-type results in fixation of the recessive allele and loss of the dominant allele. Similarly, selection against a recessive phenotype results in near fixation of the